It is known from EP0889573A1 to provide a limited free-motion coupling for a pump impeller which utilises two confronted surfaces extending laterally to the rotational axis of the rotor shaft. One surface is provided on the rotor shaft, and the other surface is provided on the impeller. Each surface includes arcuate channels formed therein. Ball bearings are provided in and project axially from the arcuate channels. When the rotor shaft rotates, the associated lateral surface rotates and the ball bearings are moved to the ends of the arcuate channels, causing the two surfaces to lock relative to each other. Consequently, following an initial angular displacement of the rotor shaft relative to the impeller, the rotor shaft and impeller become engaged, and the impeller rotates.
For a motor with low or limited start-up torque, this initial unloaded free-movement allows the angular velocity of the rotor shaft to increase prior to the motor being put under load from the impeller.
A significant problem that has been identified with the known arrangement is that the provision of arcuate channels in the lateral surfaces results in the impartation of moments through the ball bearings and perpendicularly to the rotational axis, tending to cause the ball bearings to ride up and even out of the arcuate channels. This results in axial separation of the surfaces. Axial separation of the surfaces leads to potential contamination within the coupling device by the ingress of external liquid and/or particulate matter, and once the ball bearings ride fully out of the channels, torque can no longer be transmitted to the impeller and the pump must be replaced.
Obvious modifications to the known design were investigated. Reinforcement of the axial engagement of the impeller and the rotor shaft, in an effort to retain the two confronted lateral surfaces in close proximal relationship, was researched. However, this increased manufacturing costs and was only partially successful since, over time, the ball bearings tended to wear the ends of the channels.
Increasing the angles of the end walls of the channels to be parallel with the rotational axis of the coupling device was also researched. However, again, it is the edges of the end walls of the channels which contact the ball bearings and through which the force is transmitted during lock-up. Over time, the edges of the channels wear, promoting displacement or roll-out of the ball bearings.
Consequently, following a significant amount of research and development, the present invention seeks to provide a solution to this problem.